1. Field of the Invention
The present invention relates to prevention of vertical migration of a plume of subsurface contamination in the vadose zone to the groundwater by forming and maintaining one or more desiccation layers in the vadose zone in or above the layer of contamination. These approximately horizontal desiccation layers act as an impermeable barrier to the vertical migration of dissolved contaminants.
2. Brief Description of Prior Art
The present invention is motivated by the need for deep vadose contamination remediation or immobilization to protect groundwater resources. A specific example includes the in situ remediation of high-level waste (HLW) contamination in the vadose zone found at the U.S. Department of Energy (DOE) sites such the Hanford Site and comprised of radionuclides such as uranium, plutonium, cesium, cobalt, strontium-90, and technetium99 (Tc99). Of these high-priority radionuclides, Tc99 is the highest priority concern, due to its (1) long half-life and (2) high mobility. Tc99 is important to the Hanford regulation community, because of potential health, safety, and environmental impacts. The Hanford threat as well as many other vadose contamination threats continue today due to both tank leaks and disposals to cribs and trenches, because the contamination remains in the vadose zone and is currently migrating downward towards the water table. Once this contamination reaches the because the contamination remains in the vadose zone and is currently migrating downward towards the water table. Once this contamination reaches the groundwater, the potential exists for significant contamination of the groundwater and eventually nearby surface water bodies. Groundwater clean-up is quite expensive, especially for radionuclides as well as other contaminants, and therefore it is desirable to immobilize the contamination and not allow it to reach the groundwater.
Historically, waste storage and disposal operations have left a significant inventory of contaminants (such as radionuclides) in the unsaturated zone. This vadose-zone inventory poses a significant risk to both the groundwater, and eventually surface waters, and therefore, is a high-priority concern for the site and regulatory officials. As a response to this growing threat, previous researchers have undertaken several studies of various remediation or mitigation strategies that could be used to stop or reduce groundwater contamination. These studies have looked at all types of barriers from permeable grout barriers to soil freezing techniques. In nearly all cases, the analyses conclude that these approaches will result in excessive installation costs to contain the known subsurface plumes, because these methods require close well spacing, and drilling in (1) the waste contamination, which may be radioactive, and (2) difficult subsurface conditions such as a deep vadose zone comprised of thick sections of various materials.
These alternative methods, which are summarized below, have been summarized by Kaback et al. (2005):
Vadose-Zone Perneable Reactive Barrier. Creation of a permeable reactive barrier in the vadose zone entails formation of a horizontal layer of either chemically or biologically reactive material to capture and stabilize the contaminant. It requires a comprehensive knowledge of contaminant transport rates in the vadose zone so that the barrier is active when the contaminants migrate through it. In addition, this barrier requires relatively accurate physical emplacement of material in a horizontal layer to reliably prevent contaminant transport to the water table, which is a similar challenge to the impermeable barrier emplacement (in-situ grouting) discussed below.
Vadose-Zone In-Situ Grouting. Demonstrations and field implementation of in situ grouting to contain waste in place have occurred at several sites using cement-based, viscous-liquid, and wax-based grouts. The major issues with use of this technology are the uniform delivery of the grouting fluid and validation of the effectiveness of the delivery. Applications to date have typically been either total volumetric encapsulation of the waste forms, or vertical/diagonal container wall formulations that have been proven to be flawed. A large-scale horizontal barrier has not been successfully emplaced at the scale and depth of this application. The number of boreholes required to ensure complete coverage of the grout is large and extremely costly for most applications.
Soil Flushing. Soil flushing is the in situ extraction of contaminants from the soil using water, an aqueous solution, or an organic solvent. It is applicable to a broad range of contaminants, including radionuclides. The lixiviant solution is infiltrated, sprayed, or injected into the contaminated soil zone and is followed by the down-gradient collection of the lixiviant containing the contaminants. The main obstacle for in situ soil flushing appears to be the great heterogeneity of the vadose-zone sediments, not only with depth, but also in lateral directions. The flushed lixiviant may not be contained and may cause impairment of uncontaminated volumes of the vadose zone and aquifer. When the vadose zone is thick, the infiltrated water may not fully drain for long periods of time, and therefore, require an on-site presence and active groundwater pumping program for the foreseeable future.
Energy-Based Methods. Various energy-based methods (or energy-based enhancements for desiccation) are theoretically applicable to contaminants in the deep vadose zone. These include various types of heating, as well as electro-osmosis or electro-kinetic methods. In the case of a deep vadose zone, these methods might be considered to facilitate vapor-phase water removal (or to induce the movement of liquid water and/or contaminants towards a collection system). In general, these methods will require extensive subsurface access and extremely close well spacing, which likely precludes applicability. A short summary of these methods is provided below.
Evapotranspiration covers are currently considered as the most practical solution to minimize groundwater contamination from existing deep vadose-zone contamination. However, even these covers are acknowledged to allow pore water above the acceptable regulatory levels to reach the water table and do not provide a desired redundant containment feature. The present invention provides a transformational advancement to enable immobilization and/or remediation of vadose zone contamination, where no current solution for the waste has been accepted by the regulatory community.
In U.S. Pat. No. 5,591,115, Raimondi et. al. teach a system and a method of blocking the vertical migration of subsurface contamination to the groundwater. The Raimondi system and method is comprised of (1) a drying system that is capable of removing about 75% or more of the saturation moisture from the aggregate particular substrate, and (2) a generally horizontal layer barrier layer formed and maintained below the region of contamination that has about no more than 25% of the soil moisture content of the particulate layer before drying. This dry layer acts as an impermeable barrier to the vertical migration of the dissolved contamination located above the barrier layer of dried material. The method and system of Raimondi only injects dry air into a horizontal subsurface layer below the plume of contamination. This application of a desiccation method is very specific to the vertical region below the contamination, and neither anticipates or describes the use of more than one desiccation layer used in combination or the value of positioning the layer in or above the contamination.